Our long-term goal is to elucidate the genetic networks that direct cell invasion through basement membranes, the dense, sheet-like extracellular matrix that surrounds most tissues. The mechanisms that cells employ to cross basement membranes in vivo remain poorly understood, as these invasions most often occur in complex environments that are difficult to study. We are thus dissecting the process of anchor-cell (AC) invasion into the vulval epithelium in the visually and genetically accessible model organism Caenorhabiditis elegans. AC invasion involves: (1) the attachment of the AC to the basement membrane, (2) its polarization towards the basement membrane, (3) the generation and reception of a chemotactic signal(s) that stimulates invasion, (4) the precise removal of the basement membrane and (5) transit through the basement membrane. We have discovered a novel role for the netrin pathway in directing the polarization of the AC's invasive cellular processes towards the basement membrane. We have also identified a specific isoform of the C. elegans fos transcription factor, fos-1b, which inhibits AC invasion, perhaps by blocking fos-1a activity, an isoform that promotes basement membrane removal during AC invasion. Finally, we have conducted a pilot screen using a database generated from previous whole genome RNAi screens and identified five additional genes that promote AC invasion, four of which have not previously been implicated in regulating cell invasion. Integrating cellular, genetic, and molecular approaches, our proposed work will: 1) elucidate a new role for netrin signaling in polarizing an invasive cell, 2) determine the mechanisms by which fos-1b inhibits AC invasion, and 3) characterize the function of new genes identified in our RNAi database screen that specifically promote removal of the basement membrane during AC invasion. Cell invasions through basement membranes play crucial roles during normal development and are essential for leukocyte trafficking to sites of infection and injury. Uncontrolled cell- invasive activity is also associated with a number of deadly diseases, including cancer and rheumatoid arthritis. The proposed work will advance our understanding of the fundamental mechanisms controlling cell- invasive behavior and thus has a strong potential to lead to new treatment strategies for a number of human diseases associated with unregulated cell-invasive activity.